WASHINGTON – Taliban leaders are plotting to poison humanitarian supplies for desperate Afghan civilians and blame the United States, the Pentagon disclosed yesterday.

The chilling plan was revealed on Day 18 of Operating Enduring Freedom as U.S. warplanes launched their heaviest bombing raids yet on Taliban and al Qaeda front-line forces north of Kabul.

U.S. forces are inflicting heavy casualties and setting the stage for a brutal winter campaign to starve the Taliban by choking off supplies to its troops.

The noose tightened further around Taliban leaders as hundreds of Afghan exile leaders and tribal chieftains met in northern Pakistan to put aside centuries-old rivalries and form a “grand assembly” aimed at creating a new coalition government under exiled King Zahir Shah.

As the pressure continued to mount on the Taliban, the Pentagon said it was disclosing information from intelligence-gathering sources about the possibility that the Taliban plans to poison food supplies dropped from U.S. military transport jets and trucked in by humanitarian organizations.

“We would never poison any foodstuffs. We are humane people, we want to provide humanitarian assistance to those in need,” Rear Adm. John Stufflebeam said.

Meanwhile, Pentagon sources said Taliban troops are hiding out in schools, mosques and residential neighborhoods to avoid the U.S. airstrikes, and are considering blowing up a mosque and blaming America in a bid to inflame Muslims.

For the fourth straight day, U.S. fighter jets and bombers attacked major Taliban troop concentrations near Bagram, along the front lines of the civil war with rebel groups of the Northern Alliance.

Witnesses said the day and night attacks were the heaviest yet and were taking a toll on the Taliban’s forces – with more than 100 troops killed and scores more injured.

“I’m a bit surprised at how doggedly they’re hanging on to power,” he said.

There was also less anti-aircraft fire directed at U.S. warplanes and fewer artillery shells fired into Northern Alliance positions yesterday – an indication that the Taliban’s weapons are being destroyed or are being moved, sources said.

Pentagon officials say the United States is targeting fuel depots and supply routes, with the approach of the fierce Afghan winter in mind.

“The winter will be harder on the Taliban than it will be for us. If their troops can’t get bullets, fuel or food, what can they do?” said one Pentagon official.

U.S. bombers, aided by Special Forces commandos operating with the Northern Alliance, are also targeting Osama bin Laden’s Arab fighters at Taliban positions – homing in on their radio messages before attacking, sources said.